The present invention relates generally to the inspection of structures employing eddy current techniques and, more particularly, to an eddy current probe and method for detecting flaws in structures having features which are nominally circularly symmetrical, such as aircraft skin structures fastened by means of rivets.
The safety and structural integrity of aircraft continues to be of concern to manufacturers as well as consumers. As the average age of aircraft grows, reliable and accurate inspection of all aircraft components becomes increasingly important, especially the aircraft surface and surface fasteners (rivets) .. It is thus necessary to detect cracks or other defects located below the aircraft surface, as well as defects located near the rivet.
Ultrasonic and single-probe eddy current techniques have been used for detecting aircraft surface defects near rivets. However, both methods have drawbacks. Ultrasonic signals can be difficult to interpret because of the complicated geometry and number of interfaces. Eddy current inspection may require the removal of the rivet, which is undesirable. In-field inspection is further complicated by factors such as hard-to-reach areas, human fatigue, and rough or uneven surfaces.
By way of further background, eddy current inspection is a commonly used technique for non-destructively detecting discontinuities or flaws in the surface of various components, including aircraft engine parts and aircraft skin surfaces. Very briefly, eddy current inspection is based on the principle of electromagnetic induction. In one approach, a drive coil is employed to induce eddy currents within the material under inspection, and secondary magnetic fields resulting from the eddy currents are detected by a sense coil, generating signals which are subsequently processed for the purpose of detecting flaws.
Eddy current testing for flaws in conductive materials is often done by mechanically scanning a single probe in two dimensions. For example, U.S. patent application Ser. No. 07/772,761, filed Sep. 16, 1991, entitled "An Improved Method for Inspecting Components Having Complex Geometric Shapes" describes methods for interpreting eddy current image data acquired by a single probe, particularly in the context of inspecting a high pressure turbine (HPT) disk dovetail slot.
The single probe scanning method is time consuming. Accordingly, probe arrays have been developed to improve the scanning rate, as well as to increase flaw detection sensitivity, compared to single probe techniques. For example, General Electric High Density Interconnect (HDI) technology has been used to fabricate flexible eddy current probe arrays. In particular, Hedengren et al. application Ser. No. 07/696,455, filed May 5, 1991, the entire disclosure of which is hereby expressly incorporated by reference, discloses a hybrid method of both electronic and mechanical scanning employing an eddy current probe array comprising a plurality of spatially correlated eddy current probe elements disposed within a flexible interconnecting structure which may be employed to collect a discrete plurality of spatially correlated eddy current measurements for non-destructive near surface flaw detection.
HDI fabrication techniques which are advantageously employed in the fabrication of the flexible array structure of the above-incorporated application Ser. No. 07/696,455 are disclosed in Eichelberger et al. U.S. Pat. No. 4,783,695, entitled "Multichip Integrated Circuit Packaging Configuration and Method" and related patents and applications such as Eichelberger et al. application Ser. No. 07/864,786, filed Apr. 7, 1992, which is a continuation of application Ser. No. 07/504,769, filed Apr. 5, 1990, now abandoned, entitled "A Flexible High Density Interconnect Structure and Flexibly Interconnected System", the entire disclosures of which are hereby also expressly incorporated by reference.
Mechanical scanning can be entirely eliminated by employing static scanning and a suitable probe array. Thus, a variety of static scanning approaches have been proposed in the literature, whereby a two-dimensional array of sense elements is placed in a stationary position, and scanning is accomplished by electronically switching between the elements. Examples of this approach are disclosed in the following literature references: Bert A. Auld, "Probe-Flaw Interactions with Eddy Current Array Probes", Review of Progress in Quantitative NDE 10, edited by D. O. Thompson and D. E. Chimenti (Plenum Press, News York, 1991), pages 951-955; Yehuda D. Krampfner and Duane D. Johnson, "Flexible Substrate Eddy Current Coil Arrays", Review of Progress in Quantitative NDE 7, edited by D. O. Thompson and D. E. Chimenti (Plenum Press, New York, 1988), pages 471-478; and Mirek Macecek, "Advanced Eddy Current Array Defect Imaging", Review of Progress in Quantitative NDE 10, edited by D. O. Thompson and D. E. Chimenti (Plenum Press, New York, 1991), pages 995-1002.
In the particular context of the subject invention, the mechanically scanned eddy current probe disclosed in Cecco et al. U.S. Pat. No. 4,808,927, entitled "Circumferentially Compensating Eddy Current Probe with Alternately Polarized Receiver Coil", and in the literature reference V. S. Cecco and F. L. Sharp, "Special Eddy Current Probes for Heat Exchange Inspection", Proceedings of the 8th International Conference on NDE in the Nuclear Industry, Kissimmee, Florida, Nov. 17-20, 1986, pp. 169-174, deserves mention. The Cecco et al. probe is for inspecting tubing from the inside, and employs a large drive coil and an even number of sense coils which are electromagnetically polarized alternately and which are serially connected.